1. Technical Field
The invention relates to a method for producing a bonded body by using diffusion bonding.
2. Related Art
A bonded body of a ceramic body made from a ceramic material and a metal body made from a metal material is used in, for example, an electrically heated catalytic converter (EHC), a temperature sensor element and a ceramic heater etc.
In such a bonded body, the ceramic body and the metal body are bonded to each other using various bonding methods.
For example, Patent document 1 (Japanese patent application publication No. 2012-76937) discloses a bonded body made by brazing the ceramic body and the metal body with a brazing material therebetween. However, since the brazing material has a comparatively low melting point, when the bonded body is used under a high temperature environment such as inside an exhaust pipe of the a vehicle, creep resistance of the brazing material becomes a problem, and the allowable working temperature of the bonded body is limited.
Patent document 2 (Patent application publication No. 2005-343768) discloses a bonded body made by heating the ceramic body and the metal body to perform diffusion bonding. This method allows diffusion layers at the interfaces of the ceramic body and the metal body to form, which causes bond portions of both bodies to have higher melting points. Accordingly, when the bonded body is used under high temperature environment, heat resistance can be secured adequately.
If diffusion bonding between the ceramic body and the metal body is performed simply, however, there is the following problem.
That is, a thermal expansion coefficient of a ceramic material composing a ceramic body is generally smaller than that of a metal material composing a metal body. Accordingly, if the bonded body is used under a high temperature environment, heat stress (tensile stress) due to the difference between the heat expansion coefficients of the metal body and the ceramic body occurs in the ceramic body, this might cause damage such as cracks to occur in the ceramic body or the bond portions of both bodies.
For solving this problem, the following method is possible. That is, between the ceramic body and the metal body, an intermediate layer whose thermal expansion coefficient is between those of the ceramic body and the metal body is provided, thereby decreasing the heat stress due to the difference between the ceramic body and the metal body. Diffusion temperatures, however, might be different between diffusion bonding of the ceramic body and the intermediate layer and diffusion bonding of the metal body and the intermediate layer. In this case, keeping each bond portion at a respective diffusion temperature is needed.